The present invention relates to deepwater platform foundations. More particularly, it relates to a tension leg platform foundation anchored to the ocean floor through a plurality of piles.
As used herein, a “tension leg platform” or TLP refers to any buoyant structure tethered to the ocean floor through substantially vertical tendons tensioned to draw the buoyant structure below its normal floating draft. Various embodiments include a full scale TLP having full drilling facilities, a tension leg well platform (“TLWP”) having only a scaled down “completion” rig, a tension leg well jacket (“TLWJ”) designed to accept well operations from an auxiliary vessel, or any other tendon deploying variation.
Tendons connect the buoyant hull to a foundation system at the ocean floor and are tensioned to draw the buoyant hull below its normal floating draft. The tendons transmit this static load to the foundation system. Further, the tendons must transmit this static load while subject to additional loads which have significant cyclical components driven by environmental forces of wind, wave and current on the hull and tendons. The combined load is transmitted to the ocean floor through the foundation system.
Some early designs for vertically moored platform concepts contemplated using the same tubular members simultaneously for the structural mooring elements and for the risers through which drilling and production operations were to be conducted. However, this was found to be impractical due to both operational constraints and the risks, difficulties, and expense of designing the tubular goods for the internal pressure in these members as flowlines and the axial load as mooring members. Thus, in application, the designs have developed with separate risers and tendons.
The bottoms of the tendons are secured to a foundation system at tendon receiving load connections or tendon receptacles. In traditional practice, the foundation system is built around a foundation template. The template is a framework which permanently interconnects the tendon receptacles and the pile sleeves. Vertical (surface) access of tendons and piles to tendon receptacles and pile sleeves, respectively, is provided by a horizontal offset therebetween in their position on the template.
In the conventional practice, the foundation template is placed and the piles are installed through the pile sleeves and set deeply into the sediment at the ocean floor. The piles are then secured to the pile sleeves and the foundation template is ready to accept tendons.
The foundation template serves two purposes in such a foundation system. First, it provides spacing and modular placement of the pile sleeves, the tendon receptacles, and often a plurality of well guides. Second, the template is a permanent fixture providing load bearing interconnection between piles anchored to the ocean floor and tendon receptacles.
However, the tendon-to-receptacle, to-template (and over)-to pile sleeve, to-pile, to-ocean floor load path of the conventional template based foundation system is an inefficient load transfer scheme. This also commits a large quantity of steel to the template and creates handling difficulties for transporting and deploying the massive template. Further, the lateral spacing between the tendon receptacles and the pile sleeves which introduces these inefficiencies also exacerbates the fatigue response of the template based foundation system.
A plurality of smaller corner templates have been used in designs which provide well guides outside of the template as an alternative to a unitary template which includes well guides. This does reduce the material requirements, but does not alleviate the inefficiencies in load transfer discussed above.
Thus, there remains a clear need for a TLP foundation system which provides an improved and more direct load transfer between tendons and the ocean floor.